Ask HN: Why isn't there a sound-only YouTube option?

I know that question should probably be directed to Youtube, but I guess they already thought about it and ruled it out.

What do you think are the reasons? There are many videos that require no sound, or that people just listen in background. There are good opportunities to save a lot of bandwidth no?

I know there is an extension that allows it, but it's less practical, and still strange that Youtube doesn't allow it